1. Field of the Invention
The invention relates to a control apparatus of a variable valve system that performs variable control of valve timing of intake and exhaust valves in an internal combustion engine, and variable control of the operation angle of those valves.
2. Description of the Related Art
In recent years, variable valve systems that vary the valve characteristics of intake and exhaust valves have been put into practical use as systems applied to internal combustion engines mounted in vehicles and the like. In internal combustion engines that employ such variable valve systems, performance with regard to engine output, fuel efficiency, and emissions and the like is improved by appropriately adjusting the valve characteristics of the intake and exhaust valves according to the operating state of the engine.
Also in recent years, variable valve systems that perform variable control of both the valve timing and the operation angle of the intake valves have also been put into practical use. In such variable valve systems, the valve closing timing of the intake valves will be earlier if the operation angle is increased and the valve timing remains fixed, and the valve opening timing of the intake valves will be later if the operation angle is decreased and the valve timing remains fixed. Therefore, in such variable valve systems, the valve timing of the intake valves is retarded according to the increase in the operation angle, such that the valve opening timing of the intake valves does not become too early or too late according to a change in the operation angle. Therefore, the valve lift curve of the intake valves in an internal combustion engine that employs such a variable valve system varies generally in the manner shown in FIG. 6.
Also, in such a variable valve system, if the operation angle is increased while the valve timing is advanced and the center of lift of the intake valve (i.e., the point at which the intake valve is fully open) is near top dead center (TDC) of the piston, the piston and intake valve may interfere with one another and so-called valve stamp may occur. In order to avoid valve stamp, the valve timing of the intake valve is also retarded according to the increase in the operation angle.
FIG. 7 is a view illustrating the relationship between the valve timing and the operation angle of an intake valve in such a variable valve system. The region indicated by hatching in the drawing is an interference region A where valve stamp occurs. Therefore, with a variable valve system such as that described above, the control range of the valve timing and the operation angle is limited to the region other than the interference region A, i.e., the non-interference region.
Incidentally, even if an attempt is made to control the valve timing and the operation angle in the non-interference region, there is a probability that the valve timing and the operation angle will temporarily enter the interference region A in a certain transient state while they shift toward their target values. For example, the valve timing and the operation angle may enter the interference region A if the valve timing is retarded late with respect to an increase in the operation angle or if the operation angle is reduced late with respect to the advancing of the valve timing.
Therefore, Japanese Patent Application Publication No. 2006-029159 (JP-A-2006-029159) proposes a control apparatus of a variable valve system as technology that attempts to more reliably avoid valve stamp. The control apparatus described in JP-A-2006-029159 estimates the valve timing and reduces the operation angle when the estimated value exceeds a predetermined value. That is, the control apparatus described in JP-A-2006-029159 sets a critical curve C somewhat below a boundary B between the interference region A and the non-interference region in FIG. 7. When the point of intersection between the current operation angle and the estimated value of the valve timing crosses the critical curve C and enters the interference region A, a valve stamp avoidance operation that involves reducing the operation angle or retarding the valve timing is executed.
In this kind of control apparatus of a variable valve system, valve stamp can reliably be avoided by setting the critical curve C far enough away from the interference region A. However, in order to be able to avoid valve stamp in all conceivable situations, the critical curve C must be set far away from the boundary B, so the control range of the valve timing and the operation angle ends up being limited by that amount. Incidentally, this problem may also occur in a variable valve system that performs variable control of both the valve timing and the operation angle of an exhaust valve as well.